A significant amount of effort has been placed on developing new methods and systems for providing energy from resources other than fossil fuels. Bio-based feedstocks are a resource that show promise as a renewable alternative source of hydrocarbons for producing fuel and chemicals.
Bio-based feedstocks including carbohydrates and “biomass” are materials derived from living or recently living biological materials. One type of biomass is cellulosic biomass. Cellulosic biomass is the most abundant source of carbohydrate in the world due to the lignocellulosic materials composing the cell walls. The ability to convert biomass to fuels, chemicals, energy and other materials is expected to strengthen the economy, minimize dependence on oil and gas resources, reduce air and water pollution, and decrease the net rate of carbon dioxide production.
There are many challenges to overcome in developing processes of converting carbohydrates to higher hydrocarbons suitable for use in transportation fuels. For example, the processes used are costly and complex making it difficult to compete with the use of traditional resources, such as fossil fuels. U.S. Patent Application Publication No. 2007/0142633 (Yao et al.) refers to a process for the conversion of carbohydrates to higher hydrocarbons. An ion-exchange resin is provided to convert the carbohydrates into usable reaction products. The reaction products are hydrogenated, and then contacted with a zeolite containing catalyst to form higher hydrocarbons. The conversion of carbohydrates to hydrocarbons in this system results in increased loss of desirable hydrocarbon products due to the formation of unwanted byproducts, such as coke, carbon dioxide, and carbon monoxide. Thus, another challenge for promoting and sustaining the use of biomass is the need to eliminate the formation of undesirable byproducts such as carbon monoxide, carbon dioxide, and coke. A further challenge is to complete the conversion of carbohydrates to higher hydrocarbons in a limited number of steps, to obtain high yields with minimal capital investment.
Current methods for converting sugars to fuel proceed through a biological route, such as yeast fermentation, to ethanol. However, ethanol does not have a high energy density when compared to standard transportation fuels. Currently, there is a need for the creation of liquid biofuels of greater energy density than ethanol, which can make use of existing fuel infrastructure. Moreover, what is needed is a method and system that provides efficient and high yield production of higher hydrocarbons suitable for use in transportation fuels and industrial chemicals from bio-based feedstocks while avoiding or minimizing the production of unwanted by-products.
Efficient conversion of carbohydrates to glycols such as ethylene glycol or propylene glycol at high yield for use as chemical products or intermediates, has been limited by the further reaction of glycols to monooxygenates and ultimately alkanes, such that high yields of glycols cannot be obtained at high conversions of the feed carbohydrates. Use of high temperatures to increase conversion in a single reaction step can also lead to heavy ends byproduct formation from carbohydrate feeds such as biomass or soluble sugars, due to reactivity of sugar intermediates to form caramelans and tars, prior to stabilization by hydrogen. Alternate use of low conversion at lower temperatures in a single reaction step requires separation and recycle of a large stream of unconverted carbohydrates, which increases processing costs for a process which targets only glycols as the principal commercial product. Low conversion to maximize glycol yields in a single reaction steps also results in the presence of unconverted feed carbohydrates including sugars and sugar alcohols, and polyoxygenated species containing more than three oxygens. These components cause excessive coke formation and deactivation of condensation-oligomerization catalyst, upon attempted processing of the monooxygenates-rich stream obtained after glycols separation, to liquid fuels.